Abstract/Summary

The Earth’s interior can be imaged by analysing the records
of propagating seismic waves. However, the global array
of permanent seismometers that record seismic energy is
confined almost exclusively to land-based sites. This limits
the resolution of subsurface images, and results in relatively
few local measurements from areas of great geological
and tectonic interest (for example, the mid-ocean ridges
and the Tibetan plateau)1. Here we use an unconventional
form of seismic interferometry2–5 to turn earthquakes into
virtual seismometers located beneath the Earth’s surface.
Seismic waves generated by one earthquake lead to transient
strain in the subsurface at other locations around the
globe. This strain can be quantified from seismograms of
independent earthquakes that have occurred in those locations.
This technique can therefore provide information on the
subsurface strain in regions of the globe that lack instrumental
networks. Applying our method to earthquakes in Alaska
and the southwestern United States, we show that the
information that can be obtained from these earthquakes
about other such events is consistent with that provided by
instrumental seismometers. Our approach may allow realtime,
non-invasive, subsurface seismic strain monitoring,
particularly in areas remote frominstrumental networks.